In the sense of WILLING, we can say
I am happy to do so.
To stress it, we say
I am more than happy to do so.
But MORE THAN HAPPY seems to be a frozen phrase, and HAPPY cannot be replaced arbitrarily.
If I want to stress 'it is normal that....', apart from adding some adverbs like ABSOLUTELY NORMAL, how can I do?
Do you mean I can't emphasize the meaning in any way?
Sure - that's completely normal.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.